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There is no trace in the fully-developed Ascidia of a chorda dorsalis, or internal axial skeleton. It is the more interesting that the young animal that emerges from the ovum HAS a chorda, and that there is a rudimentary medullary tube above it. The latter is wholly atrophied in the developed Ascidia, and looks like a small nerve-ganglion in front above the gill-crate.
Pétersbourg," Ser. vii, Vol. 19, No. 2, tells us that the nerve-ganglion of the ascidia lies on the side of the stomach, and on that account can not be homologous with the spine of the vertebrates, but that the cord in the larva of the ascidia is nothing more than a support for the tail in swimming, which afterwards disappears, as with many other larvæ.
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